The 1950s-1970s saw the discovery of multiple classes of antibiotics, and their development into drugs changed a simple bacterial infection from life threatening to trivial. This golden age of antibiotics engendered such optimism that it was commonly thought bacterial infections would be rapidly eliminated as a cause of mortality. Unfortunately, bacterial resistance to all classes of antibiotics soon appeared. Now, three decades after the end of this era, drug-resistant bacteria are ubiquitous in hospital settings and annually 90000 people die of such infections each year in the US alone. One quarter of the bacteria that most frequently cause hospital-acquired infections are resistant to the preferred antibiotic treatment, and an alarming 70% of hospital acquired infections are resistant to at least one antibiotic.
Methicillin-resistant Staphylococcus aureus (MRSA) is the most common drug-resistant bacteria in hospitals, accounting for greater than 30% of all nosocomial infections. MRSA can also be community-acquired, causing severe illness and even death. Furthermore, the incidence of extended spectrum-lactamase (ESBL) production in clinical Klebsiella isolates has increased steadily in the past several years, severely curtailing the effectiveness of lactam antibiotics. Perhaps most disturbing is the recent estimation that one third of enterococci in intensive care units are resistant to vancomycin, often viewed as the antibiotic of last resort.
The list of drug-resistant pathogens is extensive and growing. These bacterial infections are of particular concern in elderly, infirm, or immuno-compromised patients. Individuals with tuberculosis, AIDS, salmonellosis, gonorrhea, or malaria who contract drug-resistant bacterial infections experience longer hospital stays and have mortality rates more than twice as high as those with antimicrobial-susceptible infections. Thus, resistant bacteria not only complicate medical treatment, but also add billions of dollars to medical costs every year.
The problem of bacterial resistance to antibiotics is exacerbated by the downward trend in antibacterial discovery and development. There has been a 56% decrease over the last two decades in the annual number of antibiotics approved by the FDA. In fact, only six antibiotics produced by large pharmaceutical companies are currently in late stage clinical trials, and all of these are derivatives of known antibiotics. Although the reasons for the halting of many antibacterial programs at major pharmaceutical companies are myriad, the acute (not chronic) nature of most bacterial infections and the public expectation for no side effects has made antibacterial research less profitable and more difficult when compared to other disease areas.
The research activity in this area has mainly concentrated on the design and in vitro studies of amphiphilic helical beta-peptides with antimicrobial activity. In view of their high propensity for helical conformations as well as their resistance to proteolytic degradation, beta-peptides represent promising antibacterial candidates.
Antimicrobial α-peptides that adopt cationic amphiphatic α-helical structures upon binding to cell membranes (e.g., mellitin from bee venom, magainins from frog skin, cecropins from porcine small intestine) are ubiquitous in nature (Antimicrobial Sequences Database, http://www.bbcm.units.it/˜tossi/pag1.htm) and represent important effectors molecules of innate immunity. These peptides generally cause cell death by a two step mechanism involving interaction with the lipid component of the membrane (while in bacteria the outer leaflet of the membrane is essentially composed of lipids with negatively charged phospholipid headgroups, it is almost neutral in plants and animals) followed by membrane permeabilization. Several mechanisms for membrane permeabilization have been postulated including transient pore formation (“barrel-stave model) or detergent-like disruption of the membrane (“carpet model”). The lytic activity of amphiphilic antimicrobial peptides is strongly influenced by properties such as helix stability, amphiphilicity (hydrophobic moment), hydrophobicity, relative width of the hydrophilic and hydrophobic faces of the helix as well as net charge. Despite many structure-activity studies, lead optimization remains challenging because sequence modifications of α-peptides generally affect several parameters at the same time. In addition, low activity on human cell membranes is a prerequisite for a cell-lytic peptide to be of therapeutic value and de-novo design of helical membrane-lytic peptides with high membrane selectivity necessitates an even finer tuning between these five different parameters.
Both the 314 and 2.512 helical backbones have been found suitable for the design of antimicrobial β-peptides. In order to cluster polar residues on one face of the helix, amphiphilic 314-helical β-peptides have been constructed from hydrophobic-cationic-hydrophobic- or hydrophobic-hydrophobic-cationic residue triads.
Some β3-nonapeptides with a C-terminal amide were found to be active against Gram-Positive (S. Aureus and E. Faecium strains were clinical isolates resistant to penicillin and vancomycin, respectively) and Gram-negative bacteria with MIC values (in the range 1.6-12.5 mg/mL) comparable to that of the antimicrobial α-peptides melittin and [Ala8,13,18]-magainin II amide (a highly potent synthetic analogue of natural magainin II). Although some of these compounds show no helix formation in water, they generally display a maximimum helicity in 40% aqueous TFE, a solvent system which is also known to promote helicity of amphiphilic α-peptides.